The present invention relates to a method and an apparatus for continuously hot-dipping steel strips, wherein a pretreatment of the steel is improved for plating.
A hot dipping apparatus for steel strip is provided with a pretreating device for removing an oxide film on a steel strip before plating, thereby improving wettability of the steel and preventing the formation of bare spots on the steel strip. In the pretreating device, the steel strip is heated to anneal and remove combustible impurities at the same time. Thereafter, the steel strip is reduced by reducing gas.
However, in cases where the steel contains easily oxidizable elements such as chromium, silicon and aluminum, such steel being for example stainless steel, the oxide film can not be removed by the reducing gas. Consequently, the pretreated steel strip has low wettability.
The hot dipping is performed by an intermetallic reaction, and the reaction is influenced by the easily oxidizable elements. Therefore, defects such as bare spots are liable to occur on the steel containing easily oxidizable elements.
Such a steel strip containing easily oxidizable elements can be plated by vacuum deposition, if the strip is heated.
If the steel strip contains easily oxidizable elements of the steel strip can be electroplated before the hot dipping, the wettability of the strip can be improved, thereby providing a product without defects. However, such a method renders the manufacturing process complicated, thereby increasing the manufacturing cost.